Many electric heating structures of domestic appliances include a heating element with a positive temperature coefficient (PTC), such as a thick-film resistive heating element. In a PTC heating element, the electric resistance increases with the temperature.
A problem of such heaters is that, in operation, the heating power of the heating element reduces as its temperature rises. The increase of the electric resistance as the temperature rises causes a reduction of the current through the heating element, and accordingly of the rate at which electric energy is converted into heat by the heating element.
For instance, in silver based heating elements the electric resistance typically increases by at least 0.2% of the room temperature resistance per ° C. This results in a power drop of more than 50% when heating up from room temperature to an operating temperature of the heating element of 250° C. and more than 20% when heating to 100° C. In a heating structure that has for instance been designed for a maximum power consumption of 2000 W, in order to avoid exceeding the maximum power usually available for household use without causing safety fuses or circuit breakers to trip, the maximum power available at 250° C. is therefore less than 1000 W. The high temperature power reduction generally associated to heaters with PTC heating elements causes an increase of the time required for heating to the maximum temperature associated to the selected or pre-set temperature setting, which manifests itself in particular during re-heating such as occurs for instance in thermostatically controlled heaters. Alternatively or in addition, the reduction of the available amount of power at higher temperatures results in a reduction of the conversion rate of a process, such as for instance steam generation, driven by the heating element.